Response of sunflower to boron supply at low root zone temperature

Low root temperature has previously been reported to induce boron (B) defic iency in some species but the mechanism is not entirely clear. Using B-spec ific resin to buffer B concentrations in solution, we found that decreasing root zone temperature (RZT) from 27 to 12 degrees C for 6 days depressed s unflower (Helianthus annuus L.) dry matter, root length, root to shoot rati o and B uptake. Boron uptake into the shoots was depressed at 12 degrees C RZT to a greater relative extent than dry matter. Furthermore, B partitioni ng to young leaves decreased with low RZT, particularly at 12 degrees C RZT . The external B requirement for shoot growth increased with low RZT. Leaf functioning was impaired by low B supply as measured by K+ leakage from the youngest mature leaf blade, whereas it was much less affected by RZT, and there was no effect of RZT on B - adequate plants. By contrast, root functi on was impaired more by low RZT than low B. Therefore, we predict that high er rates of B application will be required for plant growth when soil tempe rature is below a critical threshold, which for sunflower was about 17 degr ees C. Conversely, practices which increase soil temperature above the thre shold may also improve plant B nutrition.